1. Field of the Invention
The present invention pertains to safety systems and more particularly to electric control circuits for safety systems used in conjunction with loading materials that may be hazardous to personnel when handled directly such as hot asphalt.
2. Related Prior Art
In modern road construction, asphalt is a material of choice due to its ability to provide a smooth surface for travel while providing a surface rough enough for tire gripping ability for stopping. Its relative cost effectiveness, durability and ease to work with provide added benefits that increase its desirability.
Asphalt, in a strict sense, is obtained as a residue from certain petroleums and primarily consists of a mixture of hydrocarbons and varies from hard and brittle to plastic forms. It is insoluble in water but soluble in gasoline. It melts on heating and burns with a smokey flame. Asphalt may be widely used for paving, roofing, paints and varnishes. Light renders certain grades of asphalt insoluble in oil of turpentine, and hence, they are used in photomechanical work. Most native or naturally occurring asphalt is a residue in the form of evaporated petroleum. For the purposes of this patent, however, the term asphalt will be used in conjunction with the type of asphalt referred to in road construction that is used for pavements.
In present day society as more and more of our road systems, including interstate super highways, are being made of asphalt, the demand for asphalt increases along with its manufacture and concomitant transportation from the location of preparation to the location of ultimate use. The term asphalt when used to describe a road, such as an asphalt road, is generally referred to as hot mix asphalt (HMA) pavement in the industry. Hot mix asphalt is a combination of aggregates and asphalt cement.
Asphalt cement is the black, sticky substance that is produced by petroleum refineries. This substance is the residue remaining after all the more valuable fractions of a barrel of oil have been removed, fractions such as gasoline, fuel oil, diesel fuel, aviation fuel and others. This sticky substance is the glue that holds pavements together. In general, this glue is only about eight percent, by weight, of the pavement.
Aggregates account for about ninety-two percent, by weight, of the mixture that goes into the pavement. The aggregates are usually various sized stones, dust and sand. Basically, the aggregates are any hard, inert mineral materials used for mixing in granulated fragments (crushed stone).
An asphalt plant is no more than a mixing facility that mixes hot asphalt cement and hot aggregates into hot mix asphalt. At an asphalt plant, paving aggregates are dried and heated, then mixed and coated with asphalt cement. This hot mixture is then placed in storage silos for short term storage.
There are two basic types of asphalt plants or HMA facilities, a drum mix facility and a batch mix facility. In a drum mix facility aggregates are stored in cold feed hoppers. The aggregates are measured into specific portions according to the kind of pavement required and carried by conveyor or a conveyor belt to a drum where they are dried and heated by a large burner. In this process pavement may be recycled and may be added to the center of the drum. Asphalt cement is stored in a holding tank where it is heated to between 275.degree. F. and 300.degree. F. The heated asphalt cement is pumped into the drum in liquid form where it mixes with and coats the aggregates. The drum operates like a clothes dryer. As it rotates, flights along its sides keep the aggregates tumbling and dropping which guarantees that they are thoroughly dried, heated and coated with asphalt cement. The coated aggregates are then carried by conveyor to the top of the storage silos.
A batch mix facility differs from a drum facility only in terms of where the asphalt cement coating takes place. In a batch mix facility, the aggregates are dried in the drum, but are then conveyed to a mixing tower where the dried aggregates are separated by a vibrating screen and dropped into individual storage bins by size. The separated aggregates are dropped from the holding bins to a weigh hopper, the amount of each size being determined by the type of mix being produced. From the weigh hopper the aggregates are transferred to a pugmill where they are coated with the asphalt cement which has been weighed separately. The finished product is traditionally transferred to a waiting truck, although it is becoming more common to transfer the mix to storage silos as is done in the drum facility.
Large quantities of asphalt are loaded into huge silos which can contain several tons of hot asphalt. In general, several of these silos are set up in line to hold a large batch of asphalt. Due to the massive quantities of asphalt needed for installing a road, many silos will exist in a single row. In general, the silos are spaced close together to reduce heat loss. The silos are all elevated a significant distance above the ground in order that a truck to be able to pull underneath them. Once a truck is pulled underneath the proper silo, a hatch is opened and the hot asphalt is loaded into the back of the dump truck. The dump truck then proceeds to transport the hot asphalt to the location where the road is being installed. The hot asphalt is unloaded from the dump truck into a spreader which will deposit asphalt across a single lane of a highway at a uniform depth. The hot asphalt is then anchored in place or compressed through the use of a steam roller. Once the asphalt is cooled it becomes a hard surface upon which automobiles and trucks may travel smoothly.
When the asphalt is being loaded from the silo into the dump truck, the dump truck pulls up with its bed directly below the hatch for the silo. Once the truck is in position, the hatch opens and several tons of hot asphalt come streaming into the bed of the truck. In general, the driver will remain seated in the truck during the loading operation to reduce the total loading time. It is essential that the asphalt be transported to the location at which the road is being built in a short period of time. Although the asphalt will remain hot for several hours, it must be place in the spreader and rolled before it cools in order to provide a strong, hard surface upon which traffic may travel. Cool or cold asphalt does not bind together very well and will deteriorate and result in potholes in the smooth surface. As a result, asphalt truckloads that arrive with the load having a temperature below the minimum acceptable are sent back for reheating or recycling.
As a result, a dump truck arrives at the plant, quickly pulls under the silos, is loaded with asphalt and the truck departs. During the loading process, the driver stops only to place or confirm the order, to have the truck loaded and to have paperwork processed. The entire loading process is done as quickly as possible to have the asphalt arrive at the work site within preparation temperature limits.
In order to reduce heat loss while the asphalt is sitting in the silo, the silos are generally constructed to have a greater height than surface width. Since heat rises, a smaller surface area will allow less heat to escape. Accordingly, the silos are placed close together so that minimal heat is lost through the sides. This results in the silos being spaced apart approximately one half dump truck length. This means that while a dump truck is positioned such that the center of its bed is underneath one silo hatch, the cab of the truck will be located approximately beneath the hatch of another silo.
This situation provides a limited, but very real, degree of danger to the truck driver. Although millions of dump trucks have been loaded with hot asphalt since the beginning of its use for roads, the occurrence of injury to human workers has been minimal. However, on very rare occasions, through some type of inexperience or some type of miscommunication or a combination of both, injury has resulted to truck drivers. For example, the unloading of several tons of hot asphalt on the cab of a truck can result in severe burns to the driver. Even though the occurrence of such injury is extremely rare, if it could be prevented, such steps should be taken.
While attempts may be made to clearly indicate which silo is to be opened, there is always the probability that a new driver unfamiliar with the loading system may be present. A light may be present to indicate the silo to be unloaded, however unfamiliarity may cause a question as to whether the light indicates where the front of the truck should be located or where the bed of the truck should be located or where the cab of the truck should stop. If the truck is positioned incorrectly, hot asphalt can be dumped on a portion of the truck which may endanger the driver.